Preserving Polymorphic Crystalline Structures in Bulk Heterojunctions for Efficient Organic Solar Cells

The crystal structure of a material profoundly influences its physical, optical, and electronic properties. However, precise control over polymorph formation of non-fullerene acceptors (NFAs) in bulk heterojunction active layers remains challenging for optimizing organic solar cell (OSC) performance. Here, we employ a non-halogenated mixed-solvent system comprising carbon disulfide (CS₂) and o-xylene (OXY) to selectively regulate the crystallization of the high-performance acceptor BTP-eC9. A specific polymorph (Form II) is preferentially induced and consistently identified in powder, neat-film, and bulk heterojunction samples by correlating wide-angle x-ray scattering (WAXS) and grazing-incidence wide-angle x-ray scattering (GIWAXS) patterns with simulations derived from the corresponding single-crystal structures. Compared with Form I, Form II exhibits improved optoelectronic properties, including extended exciton diffusion length, balanced charge mobility, and suppressed non-radiative recombination. Consequently, Form II-based devices achieve a power conversion efficiency (PCE) of 20.61% with a fill factor of 82.85%, representing a 27.0% relative improvement over Form I devices (PCE = 16.23%). Moreover, this method enables scalable manufacturing, with flexible OSCs (1 cm² active area) fabricated by slot-die coating achieving a PCE of 17.93%. This work establishes a practical pathway for solution-processed polymorph control and reveals the relationship among single-crystal packing, acceptor molecular packing, and device performance.